Concrete is capable of withstanding significant compressive loads, however, it is not as capable of withstanding significant tensile loads. Thus, it is often necessary to reinforce concrete structures with steel bars, cables, or the like to enhance the structure's ability to withstand tensile forces.
The basic principles of providing such reinforcement to concrete structures are known in the prior art. In a post-tensioned reinforcement system, several steel cables (called “tendons”) are placed within the concrete framing structure where the concrete will later be poured around them. The tendons are formed of several high tensile strength steel wires wound in a helical pattern around a centrally positioned steel wire. When the tendons are placed within the framing structure, each tendon is held loosely in place, and the ends of each tendon pass through an anchor on each end of a concrete member that composes a portion of the total concrete structure. Once the concrete is poured and has cured for a sufficient amount of time, but not yet to the point of being fully cured, the tendons may be tensioned by a hydraulic tensioner. The hydraulic jack tensioners that may be used in these circumstances are driven by high pressure hydraulic fluid in one or more cylinders in the tensioner, which places the tendon under a high tensile load, for example 30-40,000 pounds force.
A concrete anchor is typically formed as a singular body by casting, forging, or machining and includes a body portion, two generally cylindrically shaped portions, one extending from the front surface of the flange (nose portion) and one extending from the rear surface of the flange (button portion). The front surface of the flange commonly has multiple ribs to help support the force applied to the tendon after tensioning. The rear surface of the flange is used to contact the concrete or other structural surface and provide a load bearing surface during the tensioning of the tendon by the hydraulic jack tensioner. The flange portion typically includes two mounting holes so the anchor can be fastened to the concrete structure, with nails or similar fasteners. Other anchor configurations constructed of multiple bores, separate components for bore holes and concrete bearing flange portions, and with or without nose and button portions are also used.
A bore passes through the nose portion, the flange portion, and the button portion and decreases in diameter along the axis of the bore in the direction from the front surface of the flange to the rear surface of the flange. Due to this decreasing diameter, or tapering, the bore is capable of receiving a wedge that surrounds the tendon. A common taper angle for anchor bores of the prior art is 7°.
Before the concrete is poured around the tendons, each tendon must pass through an anchor that will be located on each side of where the concrete slab will eventually be located. The tendon enters the anchor by entering the bore in the button portion on the rear surface of the flange and exiting the bore in the nose portion on the front surface of the flange. After the tendon exits the anchor, the wedge may be placed around the tendon in the frusto-conical bore of the anchor.
The wedge is generally frusto-conical in shape and is usually composed of two or more segments. The internal surface of the wedge has a gripping structure for gripping the tendon. The outer surface of the wedge engages the bore of the anchor, and as such, the outer surface of the wedge generally matches the taper angle of the bore of the anchor. Therefore, wedges are constructed such that the outer diameter decreases from the front of the wedge to the rear of the wedge.
After the concrete is poured and allowed to partially cure for a sufficient amount of time, the tendon may be tensioned by a hydraulic jack tensioner. When the tendon is tensioned by the hydraulic jack tensioner, the tendon and wedge are forced tightly into the bore. The tensioning force on the tendon passes to the wedge and to the nose, button, and flange portions of the anchor, and ultimately, to the concrete slab. The ribs help distribute that force throughout the body of the anchor and onto the rear surface of the flange portion of the anchor, thus providing the tensile strength to the concrete structure.
While anchor systems and the various components that compose them have been subject to minor changes, the efficiency of anchor systems has stayed rather constant since their inception. In fact, the average overall efficiency of a current anchor system, as measured by the tensile strength at failure compared to the ultimate tensile strength of the tendon, is approximately 95%. With the widespread use of anchor systems in the construction of concrete structures, any improvement in anchor systems will help to maintain the integrity of concrete structures and lead to longer life spans for such structures. In addition, obtaining a more efficient anchorage system would prove especially beneficial for structures built in environments that have a greater likelihood of seismic activity.